In conventional telephone systems, the central office is connected to each subscriber station by a loop consisting of a pair of wires over which analog voice signals, dial pulsing and switch-hook signaling are transmitted. A power source such as the standard central office battery is connected to the loop of a conventional system and provides electrical power to the subscriber set for both voice and signal transmission. The lifting of a handset or the operation of a standard dial causes switches to be operated in the telephone set, and the resulting changes in current flow in the loop are detected at the central office which reacts in the appropriate manner.
In modern digital systems such as the Integrated Services Digital Network (ISDN), both voice and signaling are transmitted in digital form between the subscriber set and the central office over a digital subscriber line. For example, instead of relying on the detection of changes in the loop current at the central office when a station set goes to the off-hook stage, the station sends a digitally encoded protocol message to the central office indicating the off-hook state. A number of other messages, which may include information about the subscriber set and the type of call being placed are also exchanged between the central office and the customer equipment. These interface protocol messages as well as the electrical interface between the central office and customer equipment are specified in general terms by an international organization known as the International Telegraph and Telephone Consultative Committee (CCITT). To meet the standardized interface requirements, an interface unit, usually located on or near the customer premises, interconnects the transmission lines from the central office with customer sets. This interface unit includes such devices as echo cancelers and other transmission interface circuits. The CCITT specifications require that the circuitry in the interface unit be powered from the central office, in order to be independent from commercial power and to provide emergency power to at least some of the customer sets. Substantial power is required to drive the remote circuitry and the power loss in two-wire loop used in a conventional manner is such that the distance over which power can be transmitted from the central office to the interface circuit is severely limited.
It is an object of this invention to provide a power transmission arrangement from the central office to remote circuitry with a reduced power loss in order to allow the circuit to be placed a greater distance from the central office without the need of repeaters or other power boosting circuitry.